Until now, terahertz wave spectroscopy in a time domain using a single channel detector is known (Reference 1 K. Sakai, et al., “Terahertz Time Domain Spectroscopy and Imaging,” Laser Kenkyu, Vol. 30, No. 7, Jul. 2002, p. 376–384). Using this known method, a probe light has to be scanned using mechanically structured optical delay line for measuring all spectra, which makes measurement time consuming, and which makes real time measurement impossible.
Lately, a method for measuring an electric field signal of the terahertz pulse using a chirped light in a real time by a multi-channeled detector is investigated. (Reference 2: Zhiping Jiang and X. -C. Zhang “Electro-optic measurement of THz field pulses with a chirped optical beam”, Applied Physics Letters, Vol. 72, No. 16, 20 Apr. 1998, pp. 1945–1947.) In this method, an ultra-short pulsed light from a laser apparatus is separated by a beam splitter to a pumping light and a probe light. The pumping light pumps an emitter for generating the terahertz pulse. On the other hand, the probe light is stretched and chirped by a grating pair. A ZnTe crystal is irradiated by the generated terahertz pulse and the stretched and chirped light. The probe light is modulated by an electro-optic effect induced from the electric field signal of the terahertz pulse. The transmitted probe light is dispersed into a spectrum by a diffraction grating, and detected by a multi-channeled detector array. A waveform detected by the detector array is the spectrum of the probe light modulated by the terahertz pulse. A horizontal axis of the waveform is a wavelength. The positively chirped probe light is extended in the time domain, and longer wavelength delays reaching a ZnTe crystal. Therefore, substantially, light delays for a certain time, depending on a wavelength thereof. Accordingly, the wavelength can be transformed into a delay time. Subtracting the spectrum of the probe light which is not modulated by the terahertz pulse from the modulated probe light yields the electric field signal of the terahertz pulse. Analyzing the electric field signal by Fourier transform yields the frequency spectrum of the terahertz pulse.
A frequency spectral range W and a resolution ΔW of the frequency spectral measurement of the terahertz pulse are written asW˜(T0Tc)−1/2/2  (1)ΔW˜1/Tc  (2)where T0 is a pulse width of the probe light before being stretched and Tc is a pulse width of the probe light after being stretched. According to equation (1), for increasing the spectral range of the terahertz pulse (increasing W), Tc should be decreased. According to equation (2), for increasing the resolution of the frequency spectral measurement of the terahertz pulse (decrease ΔW), Tc should be increased. There is a trade-off between the frequency spectral range and the resolution of the frequency spectral measurement of the terahertz pulse. Therefore, wide spectral range and high resolution can not be obtained simultaneously.
As mentioned above, in known multi-channeled measuring method for measuring the spectrum of the terahertz pulse, when the spectral range is widen, the resolution of the measurement is lowered. On the other hand, when the resolution of the measurement is raised, the spectral range is narrowed.
A need thus exists for a multi-channeled measuring method and a multi-channeled measuring apparatus for measuring a spectrum of a terahertz pulse with both wide spectral range and high resolution.